Bleaching and delignification of partially delignified pulp with a mixture of chlorine and chlorine dioxide

ABSTRACT

A process is provided for bleaching and delignifying cellulose pulps that have previously been subjected to a partial delignification under alkaline conditions with an oxygencontaining gas, which comprises delignifying the pulp with a mixture of chlorine and chlorine dioxide, and then dissolving chlorinated lignin in a further alkaline treatment.

lJnited States Patent lCroon et al.

[451 Mar. 28, 11972 [54] BLEACHING AND DELIGNIFICATION 0F PARTIALLY DELIGNIFIED PULP WITH A MIXTURE OF CHLORINE AND CHLORINE DIOXIDE [72] Inventors: Ingemar Liss-Albin Croon; Sten Oskar Dillen, both of Alfredshem; Sture Erik Noreus, Sundasen, all of Sweden [73] Assignee: M0 Och Domsjo Aktiebolag, ()rgskoldsvik,

Sweden [22] Filed: Dec. 5, 1969 [21] Appl. No.: 882,812

[58] Field of Search ..162/65, 66, 67, 89

[56] References Cited UNITED STATES PATENTS Rerolle et a]. 162/65 3,536,577 10/1970 Rapson 2,147,618 2/1939 Rawling Primary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. Corbin Attorney-lanes & Chapman [5 7] ABSTRACT A process is provided for bleaching and delignifying cellulose pulps that have previously been subjected to a partial delignification under alkaline conditions with an oxygen-containing gas, which comprises delignifying the pulp with a mixture of chlorine and chlorine dioxide, and then dissolving chlorinated lignin in a further alkaline treatment.

16 Claims, No Drawings This invention relates to a process for delignifying cellulose that is not removed is discolored. Therefore, it is necessary to keep the amount of chlorine at the minimum that will give effective delignification, while chlorine alone is not sufficient,

and bleaching must be carried to completion using lignin by oxidation and converting it into a water-soluble or alkali-soluble form.

Chlorine is considered to be the most effective and least expensive of the delignifying agents that are available. However, it has a tendency to decompose the cellulose,

pulp that has previously been subjected to treatment with an 5 hypochlfmt? and/or oxygemcomaining gas under alkaline conditions and more Chlorination thus rs usually carr ed out in such a manner particularly, to a multi-stage process for bleaching cellulose h there a f chlonnePresem the conch" pulp which includes asalater treatment stage, followingaparslon of the This excess is i lmponam to tial delignification treatment with an oxygen-containing gas control If there F high an excess remaining f under alkaline conditions, a delignification stage with a mixdency of the Fhlonnnon Stage to detcompose cellulose ture of chlorine and chlorine dioxide at a temperature within creased: i if there excess, 1' If theexcess is too small, the range from about to about for a time withinthe the delignification of the cellulose lS incomplete. if the range from about 30 to about 300 minutes, and at a pulp conchhtmatwn must be i m t the risk decom sistency f at least 2 percent and preferably within the range position of the cellulose 15 increased in the second stage, and from about 3 percent to about 15 percent, and thereafter one 15 therefore the h 'f of the treatment Stages is usually further treatment stage under alkaline conditions to dissolve such that the chlorination is restricted w stage, and a chbrinated lignim stitu te hypochlorite or chlorine dioxide stage is used instead, if

The bleaching of cellulose pulp obtained by either the posslble' sulfate or sulfite process is a multi-stage procedure, and h chlormatlon, Stage C, is y Performed with a usually includes as separate steps in any of a larger number of 20 relatively low P p consistency from about 1 to about 4 P Sequences f one or more f each, cent, and at low temperatures, from about 5 to about C a hl i i Stage, for short treatment times, from about is to about 2 hours. The an alkali treatment or alkali extraction Stage, quantity of chlorine used varies with the properties of the unc. a hypochlorite treatment stage, and 25 bleached pulp, but is sufficient only to give an excess within d a hl i i id treatment Stag; the range from about 0.1 to about 0.5 percent chlorine, based These treatments can be carried out in almost any order, and Oh the Weight of the y P p, at the Conclusion of the treattwo or more of such stages are frequently included, but usually mentnot in succession, but i te lea d ith one or more f h Chlorine dioxide can also be used in place of chlorine in the other stages. chlorination stage. In this event, the amount of chlorine dioxin general, sulfate pulps require more treatment stages for ide used is sufficient to supply an amount of chlorine within complete bleaching than sulfite pulps. Also, in the case of the range set out above sufficient to ensure an excess of from sulfate pulps, chlorination, alkali treatment and chlorine dioxto pe n Ch in at th nd f th tr atmentide treatment a e lmo t lwa necessary hil h The alkali extraction, stage E, is designed to remove the lighy ochlorire t atment a o eti e be mitted, wherea i nins that have been made alkali-soluble in the chlorination the case of, sulfite ul eithe the hypochlorite or the stage, and is carried out at a relatively high pulp consistency. chlorine dioxide treatments can be omitted, and the bleaching T is no m ly at from about 4 to about 13 P when can be reduced to as little as three or four stages. However, all using presses for dewatering, up to 30 percent. The temperamulti-stage bleaching processes employ a chlorination stage ture n be rather from to but in the so-called and an alkali treatment stage, and the reason is that these hot alkali refining processes, for producing pulp with a high stages are required for delignification. Some sulfate pulps are alpha cellulose content, temperatures of up to 130 C. can be more difficult to delignify than others, and in the case of these employed. The treatment time can be very short, for just pulps, one chlorination stage may be insufficient to delignify several minutes, up to from four to five hours. The amount of the pulp sufficiently. Hence, a complementary delignification alkali charged to the system varies according to the properties stage can be carried out in an additional chlorination stage, or i of the pulp which it is desired to produce. in the case of paper in a hypochlorite treatment stage, or in a chlorine dioxide pulps, it is usually carried out in such a manner that the pH at treatment stage. the conclusion of the treatment is within the range from about The conditions under which the various treatment stages 8 to about 12.5,generally about i 1. are carried out in conventional bleaching processes are sum- The hypochlorite treatment, stage H, is also carried out at marized in Table A, which follows: 7, relatively high pulp consistencies, normally from about 4 to TABLE A Pulp consistency, Tempera- Time, Stage Treatment percent ture, 0. hours Amount 0 Chlorination (C1? or 010 or both) 1-4 6-25 i Z Enough C12 or 0101 to provide at end 0! treatment 0.1 to 0.5%

v excess 012 based on pulp weight. E Alkali extraction (NaOI-I) 4-18 1 26-80 Enough NaOH to give a pH 8-12.15 at end of treatment. H Hypochlorite 4-18 25-60 1-4 From 0.2 to 3% 01 to provide at; end of treatment a 0.1% Ci: (Na 0G1 excess based on ulp weight, and alkali (normally between or :I 10 and 100% oi chlorine amount) to give a pH 8.5, usually 08(001): 10-11. excess alkali (NaOH).

D Chlorine dioxide (C102) t-lb 2-5 Enough to provide 0.5 to 6% 01:, based on pulp weight, with an excess of from 0.1 to 0.6% Ch at end of treatment. 1 Up to C. in hot alkali refining. 2 Several minutes to 4-5 hours, 7

Bleaching includes the chemical steps of decomposing the about 18 percent. The temperatures are usually rather higher than in either the chlorination or the alkali treatment, and range from about 25 to about 60 C. The treatment time is also somewhat longer, from one hour and upwards, normally from two to four hours. The quantity of hypochlorite charged and thiiis .75 l e z em ar a withlhstm tpyln #BLt n io and is based on the amount of chlorine provided. The amount can range from vary small amounts, of the order of 0.2 percent C1 by weight of the pulp, to 3 percent C1,, based on the weight of dry pulp. This treatment is also carried out in a manner to ensure that a small excess of chlorine remains at the conclusion of the treatment, about 0.1 percent by weight of the pulp.

Hypochlorite having a pH at about and immediately below the neutral point of 7 strongly decomposes cellulose, and consequently an amount of alkali is also charged to the system to provide a surplus of alkali, and maintain the pH well above 7.

The amount of alkali charged as NaOH can be within the range from about 10 to about 100 percent by weight of the chlorine charged to the system in this stage, so as to ensure that at the conclusion of the hypochlorite treatment the pH is above 8.5, and is normally within the range from 10 to 11.

The chlorine dioxide treatment is carried out with the same pulp consistency as the hypochlorite treatment, although in practice a somewhat lower consistency can be used, since the chlorine dioxide is obtained in a more dilute form than the hypochlorite solution. Thus, the pulp consistency during chlorine dioxide treatment can be within the range from about 4 to about percent. The temperature is higher, within the range from about 50 to about 90 C., and normally about 70 C. The treatment time is at least two hours, and can be from three to five hours.

The quantity of chlorine dioxide charged to the system is based on the amount of chlorine equivalent to the chlorine dioxide within the range from about 0.5 to about 3 percent C1 based on the weight of the dry pulp, and sometimes even more than 3 percent, up to about 5 percent. This treatment is also carried out in a manner to ensure a small excess, about 0.! percent to 0.5 percent C1,, at the end of the treatment.

The usual bleaching process for sulfate pulps includes six stages, in the order C E H D E D, the letters corresponding to the treatment stages set out above. An alkali extraction normally follows one or more stages of delignification, to remove solubilized lignins and other extractives. These stages can also be arranged in other combinations, for example,

C E C E H D E D C E H C H D E D C H E D E D C E D E D Sulfite pulps can be bleached with shorter bleaching sequences, and illustrative sequences include C E C E D C E H D C E D H C E H H C E H It will be noted that in all of the sequences in common use for sulfate and sulfite pulps, the first treatment stage is invariably a chlorination stage, and the second treatment stage is almost invariably an alkali or extraction stage, to remove the lignins and other alkali-soluble extractives. In the single exception noted above, the hypochlorite stage serves as a substitute second chlorination stage, seeking to reduce the risk of cellulose decomposition in the first chlorination stage. Anyhow as pointed out above also this hypochlorite stage comprises an alkali treatment immediately following the chlorination. An alkali extraction is interposed wherever more alkalisoluble extractives are to be removed.

The difficulty of control of the chlorination stage so as to prevent decomposition of the cellulose has drawn the attention of numerous workers in the pulp bleaching art, and many suggestions have been made. The most effective and least expensive alternatives, however, have involved the substitution of a hypochlorite or chlorine dioxide stage for a second chlorination stage, and a reduction in the severity of the first chlorination. Even in this variation, however, it is not possible to prevent decomposition of the cellulose altogether, particularly when the complementary stage is a hypochlorite treatment.

It has recently been proposed that the chlorination be carried out using a mixture of chlorine and chlorine dioxide. The amount of chlorine dioxide can be rather small, up to a major proportion. The chlorination is otherwise carried out in the usual way under the treatment conditions as set out in Table A above. However, the degree of amelioration obtained of the tendency to decompose the cellulose is not under these conditions sufficient to compensate for the considerable increase in the cost of the chlorination, due to the higher cost of chlorine dioxide as compared to chlorine. Moreover, the amelioration of the decomposition of the cellulose may also be accompanied by an incomplete delignification, as a result of which it becomes necessary to introduce another chlorination stage, or a hypochlorite or chlorine dioxide treatment stage, so as to complete the necessary delignification. In this event, there remains the possibility of decomposition of the cellulose during this complementary stage.

If it is desired to lower the costs of bleaching pulp, it is necessary to look for an oxidizing agent that is cheaper than chlorine. It has long been known that under alkaline conditions oxygen is capable of oxidizing lignin, partially solubilizing the lignin, and rendering the residual lignin brighter. To achieve rapid reaction between the cellulosic material and the gaseous oxygen (or air) supplied, the partial pressure of the oxygen at the beginning of the treatment should be at least one atmosphere. Lower pressures can be used, but normally an oxygen pressure of from about 2 to about 12 atmospheres is preferred. Still higher pressures can be used, but this increases the cost of reactor and of operation. At low temperatures, e.g. 50 C., the reactions proceed slowly, so that reactors of high capacity are required for large production rates. To obtain short reaction times, the treatment is normally carried out at a temperature within the range from about C. 130 C., preferably at about to 1 10 C. The consistency of the cellulosic material may be varied within wide limits without jeopardizing the reaction, e.g. between 3 and 45 percent. Treatment at low pulp consistencies of 10 to 15 percent requires a reactor in which the pulp is strongly agitated, to achieve a satisfactory delignification. If higher pulp consistencies are employed, no agitation of the pulp is required. At a high pulp consistency, the pulp should be shredded mechanically after or as the chemicals are supplied.

The amount of alkali required for the process depends upon the quantity of lignin and hemicellulose which it is desired to remove. Normally, the amount of alkali (calculated as NaOH is from about 0.5 to about 12 percent, based on the weight of the cellulosic material. Amounts of from 7 to 12 percent are needed to solubilize large amounts oflignin and/or hemiccllulose. For pulps of low lignin content, amounts of from 0.5 to 7 percent are usually sufficient. The dissolution of hemicellulose decreases as the amount of alkali is reduced.

The selectivity of the oxygen-alkali treatment for dissolution of lignin is considerably improved, if an inhibitor for the reaction of oxygen with carbohydrates of the pulp, such as a magnesium compound, is added.

However, this oxygen bleaching process has the disadvantage that even in the presence of magnesium compounds, an unacceptable degradation of the pulp results, with an undesirable reduction in viscosity.

The process of the present invention avoids these disadvantages, without excessive degradation of the pulp, and a concomitant viscosity reduction.

In accordance with the instant invention, it has been determined that a second bleaching and delignifying stage, in which a mixture of chlorine and chlorine dioxide is used (referred to herein as a C/D stage) can be arranged to follow as a first oxygen-bleaching and delignifying stage, and a later alkali extraction stage can then dissolve the chlorinated lignin, with good control of the decomposition of the cellulose pulp, as well as an at least substantially better bleaching and delignification than in prior oxygen-alkali pulp treatment processes, and in many cases, a substantially complete bleaching and delignification.

In the multistage pulp bleaching process of the invention, the first bleaching and delignification stage is carried out under alkaline conditions with an oxygen-containing gas until a partial delignification has resulted, and in a subsequent treatment stage, without intermediate chlorination, delignification is continued with a mixture of chlorine and chlorine dioxide at a temperature of within the range from about 20 to about 70 C. for a period of from 0.5 to 5 hours and at a pulp consistency of at least 2 percent and preferably between about 3 and about percent, and then the pulp is subjected to at least one further treatment stage under alkaline conditions, to dissolve chlorinated lignin.

It has been determined that in this multistage process, continuing delignification with a mixture of chlorine and chlorine dioxide, applied in relatively drastic conditions, after the oxygen-alkali treatment, and excellent delignification is obtained, under conditions which are very favorable for retaining the pulp viscosity. The good delignification effect results in a cellulose pulp which, after alkali treatment and a chlorine dioxide stage, has a brightness and a viscosity quite comparable to those of a conventionally bleached pulp, when this is bleached by a six-stage bleaching sequence. This reduction in the number of stages from six to three is of great economical importance, since the investment cost in a bleaching stage in a modern pulp factory amounts to about $400,000, to which operating and maintenance costs, of course, have to be added.

Using a fivestage bleaching schedule for oxygen-alkali treated pulp, i.e., the three-stage as described above, to which are added an additional alkali treatment stage and a chlorine dioxide treatment stage, pulps are obtained which combine high brightness and high viscosity to an extent hardly achievable by conventional bleaching processes. The tendency to brightness reversion of the pulp is also very low. Bleaching according to this invention of an oxygen-alkali treated pulp by a C/D stage and one or more chlorine dioxide stages with intermediate alkali treatment stages involves a reduction in bleaching cost (as compared to conventional bleaching methods) of the order of $2.00 per ton of dry pulp.

The chlorine-chlorine dioxide treatment and/or the final alkali extraction stage to dissolve chlorinated lignin can be followed by one or more hypochlorite (l-l) and/or chlorine dioxide (D) bleaching stages, interleaved with alkaline extraction stages (E), under conventional conditions, as shown in Table A in the multistage pulp bleaching and delignification process of the invention, the first oxygen-alkali treatment stage can be carried out in the presence of alkali in the normal way. in order to obtain a rapid reaction between the cellulosic material and the oxygen gas or air supplied to the system, the partial pressure of oxygen at the beginning of the treatment should be at least one atmosphere. However, lower pressures can be used, when a slower reaction is acceptable. When using pure oxygen gas, the process can be carried out at pressures approximating atmospheric pressure, while if air is used, because of the lower proportion of oxygen, higher pressures, usually superatmospheric pressures, are employed. If oxygen is used, a practical upper limit is 20 atmospheres, while if air is used, a practical upper limit is 60 atmospheres. The higher the pressure, the more rapid the reaction. Usually, an oxygen gas pressure within the range from about 2 to about 12 atmospheres is preferred.

It is frequently expedient to supply the oxygen gas or air during the process, and to release air enriched with regard to inert gas during the process.

The reaction will proceed at low temperatures, of the order of 25 to 50 C., but then the reaction is slow, and a large reaction vessel is necessary. Consequently, in order to reduce reaction time to a practical range, and keep the equipment small, the treatment is usually carried out at a temperature within the range from about 80 to about 150 C. If it is desired to reduce the viscosity of the pulp, the higher temperatures can be used, of the order of 130 to 140 C, When treating sulfate paper pulps, a lower temperature is used, if a significant reduction of the hemicellulose content is not desired. If a significant reduction of the hemicellulose is desired, however, then it is desirable to employ a rather high temperature. Usually, in the case of sulfate paper pulps, the treatment is carried out advantageously at from to C.

The temperature can be varied upwardly or downwardly, progressively or continuously, during the process. it is in many cases desirable to begin the reaction at a low temperature, and then to gradually increase the temperature during the reaction. This is particularly true in the case of pulps containing hemicellulose which in an unoxidized condition is attached by alkali, for example, sulfite pulps, and semichemical pulps. Thus, the reaction temperature is low while the hemicellulose remains unoxidized, but as it becomes oxidized, in the course of the reaction, the temperature can be increased, thus reducing the total reaction time.

The concentration of cellulosic material in the reaction mixture can be varied within wide limits, and is in no way critical. Concentrations within the range from about 3 to about 45 percent are employed. It is, however, preferable to effect the treatment at a concentration in excess of 10 percent, and preferably within the range from about 15 to about 35 percent. When high pulp concentrations are treated, the pulp should be shredded mechanically after or at the same time as the reagent chemicals are added to the reaction mixture.

In a preferred embodiment of the invention, which gives a particularly uniform treatment and a pulp whose properties can be controlled within the narrow limits, the cellulosic material is first impregnated with an aqueous solution of a soluble magnesium salt, or an aqueous solution of the components which in admixture give rise to the soluble magnesium salt, before being treated with air or oxygen. The excess of the impregnating solution can then be removed, for example, by filtering and/or by pressing, before the treatment is begun. The solution that is removed can, of course, be used for impregnating additional cellulosic material.

The amount of alkali required in the treatment depends on the quantity of lignin and/or hemicellulose which it is desired to remove. Normally, the alkali charge (calculated as NaOl-l) is within the range from about 0.5 to about 12 percent NaOH, based on the weight of the cellulosic material present. Other alkalis can be used, such as potassium hydroxide or lithium hydroxide, in which event the amounts are changed proportionately. If it is desired to dissolve large quantities of lignin and/or hemicellulose during the process, an alkaline charge within the range of about 7 to about 12 percent can be used. When treating a pulp having a low lignin content, in which case a smaller amount of lignin and/or hemicellulose is to be dissolved, the charge can be within the range from about 0.5 to about 7 percent.

The proportion of hemicellulose dissolved decreases as the amount of alkali is reduced, and accordingly, the amount of both the lignin and the hemicellulose dissolved can be regulated by control of the amount of alkali added.

It may be advantageous to add only a portion of the total quantity of alkali at the beginning of the process, and then add additional alkali as the reaction proceeds. The alkali attacks the lignin preferentially, and by limiting the amount of alkali present at any given time, it is possible to remove the lignin with a minimum of attack upon the cellulose and hemicellulose in the course of the reaction. The desired grade of pulp can thus be controlled by the manner and rate at which the alkali is charged to the system, and the size of the alkali charge, and the reaction time.

The alkali can be combined with the pulp either before, during, or after combination with a soluble magnesium salt, and it can be introduced in whole or in part in this way. The mixing with alkali can be effected at the desired reaction temperature, or at a lower temperature, after which the temperature is increased to reaction temperature.

The reaction time required decreases with an increased oxygen gas pressure and the reaction temperature. If the oxygen gas pressure is high, and the reaction temperature is high, thereaction can be complete in rather a short time, for example, five minutes. When oxygen gas is employed at atmospheric pressure, treatment times of 10 hours and more can be used. Normally, however, in a commercial process, where a high rate of production per hour is desirable, the reaction times will be within the range from about 10 to about 120 minutes. The reaction time is easy to control, since the reaction halts when the alkali is consumed, and thus the reaction time can be increased or shortened, depending upon the amount of alkali added at any given time, for a given gas pressure and temperature of reaction.

The delignification that is obtained in this stage is only partial. Delignification is continued in the subsequent treatment with a mixture of chlorine and chlorine dioxide under relatively severe conditions, as compared to the usual conditions shown in Table A.

The pulp consistency is rather high, and at least about 2 percent, preferably is within the range from about 3 to about percent. The temperature is also rather high, within the range from about to about 70 C., and the treatment time is within the range from about 30 to about 300 minutes. These are conditions not usually employed in a chlorination step, but correspond instead to the conditions of a hypochlorite or chlorine dioxide treatment. it is therefore possible to refer to this bleaching and delignification stage of the invention as a substitute for a hypochlorite treatment stage, or for a chlorine dioxide treatment stage, in the usual bleaching process. However, it otherwise has little in common with these latter two stages, because it is a true delignification stage as well, whereas the others are primarily bleaching stages, and only complementarily considered as delignification stages.

in this bleaching and delignification stage of the invention, the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture can be within the range from about 2 to about 50 percent chlorine dioxide, preferably within the range from about ID to about 30 percent chlorine dioxide. The amount is calculated in terms of the amount of chlorine equivalent to the chlorine dioxide.

The chlorine is employed as chlorine liquid or chlorine gas, and is added to an aqueous suspension of the pulp in any conventional way. Chlorine dioxide is also added as such, or can be formed in situ. Usually, it is preferably to prepare chlorine dioxide as and when required, and then add it directly to the aqueous pulp mixture. Chlorine and chlorine dioxide should preferably be mixed thoroughly before the addition thereof into the aqueous pulp mixture.

This bleaching and delignification stage of the invention offers several advantages over prior treatments, among them a lower cost for the chemicals required, and a very much smaller risk of decomposition of the cellulose. It is possible because of the benefits of this bleaching and delignification stage of the invention to lower the cost for the chemicals required during bleaching, because of a reduction in the amounts of the chemicals, and a rearrangement of the order of the various treatment stages of the bleaching process, with an unchanged or even improved quality of the pulp, in consequence.

An alkali extraction stage is interposed following the chlorine-chlorine dioxide treatment stage of the invention to remove the alkali soluble lignin derivatives and other alkalisoluble extractives formed in that stage. This alkali extraction can directly follow the Cl -ClO treatment, or be interposed at a later stage.

The alkali extraction can be under the usual conditions, as shown in Table A. However, lower alkali concentrations can be used, since a large proportion of alkali-soluble extractives will have been removed in the first oxygen-alkali treatment, in which some extraction also takes place. Thus, the amount of alkali can be from 0.5 to 1.5 percent, and at such amounts the pH will be from 8 to 12.5 at the end of the extraction, as in the usual alkali extraction stage.

The pulp consistency in this stage is within the usual range from about 4 to about 18 percent, the temperature within the usual range from about 25 to about 30 C., up to about 130 C. in a hot alkali refining stage, and the treatment time can range from several minutes to about five hours.

At the conclusion of the alkali extraction stage of the invention, the resulting pulp is more delignified, and has a better quality. The bleaching of the delignified pulp has been performed with less chemical consumption, and results in better quality of the final bleached pulp. Quality of the final bleached pulp is evaluated by the usual criteria: (1) brightness, SCAN before and after aging at elevated temperatures, (2) viscosity (which is determined for an aqueous copper ethylene diamine solution of the pulp according to SCAN Cl5z62, and which is a measurement of the mean polymerization degree of the cellulose, i.e., the chain length of the cellulose molecules), and (3) the extinction at 205 nm., i.e., ultraviolet light (which is determined for a solution of the pulp in phosphoric acid containing 5 mg. of pulp per ml. and is a measurement of the quantity of lignin remaining in the pulp). The higher the viscosity, the greater the strength of the pulp, and the lower the extinction, the greater the extent of delignification.

The following Examples represent preferred embodiments of the invention.

EXAMPLE 1 An unbleached cellulose pulp prepared from pine wood by the sulfate process, having a kappa number of 33.2 (the kappa number is a measurement of the lignin content of the pulp, and thus of its consumption of bleaching chemicals. The determination of kappa number is according to the standard in SCAN-C 1:59), a viscosity (SCAN) of 1,236 em /g. and a viscosity (TAPPl) 177 cp., was treated by the process of the invention.

The pulp and an addition of 1 percent by weight (based on the pulp) of magnesium carbonate and aqueous sodium hydroxide (6.4 g. NaOl-l per liter) were blended thoroughly at 3 percent pulp consistency. Thereupon the pulp was dewatered in a centrifuge to a pulp consistency of 24 percent.

The pulp was treated in a peg shredder to obtain a fluffy pulp. This pulp was added to an autoclave, which was heated with direct steam to 100 C. the autoclave was connected to an oxygen tube, and the pressure adjusted to 8 kgJcm. After a reaction time of 15 minutes, the pulp was washed with water. The pulp at this stage had the following characteristics:

Kappa number 15.9 Viscosity (SCAN) L024 cm. g. Viscosity (TAPPl) 88.2 cp

a. One portion of this pulp was bleached according to the following schedule:

Treatment Conditions b. Another portion of the oxygen-alkali treated pulp was bleached according to the following schedule:

Treatment Conditions Stage C/ D E D E D Pulp consistency, percent 6 8 6 8 6 Time, hours 2 5 Immerse:

Time, hours i l 2 5 ffigfl i Temperature, "C. 60 50 s e c D E Actlve chlorine 1 4.67 1.7

mg l D E D NaOl-I it I.5 Active chlorine, percent 4. 15 0, 9 0, 6 R800 l NBOH, percent 1. O, 9 5 (calculated as active Cl) 85Il$ Ratio Gig/C; (calculated as active Cl). 85/15 Residual chlorine, 0.14 0 05 Residual chlorine, percent 0.05 0.01 0. 04 Find p" I 75 u 2 Final pH 2. 0 11. 5 a. 5 11.1 4.1

c. Another portion of the same pulp was also bieEEHd (as a control) according to the conventional bleaching 10 Th f ll i l i d r b i d f the pulp: schedule C-E-H-D-E-D, without preliminary oxygen-alkali treatment of the pulp. This portion of the pulp was bleached under the following conditions:

- Brightness (SCAN), 93.0

. l 5 Brightness after aging Treatment Conditions 16 hum C a Stage C E H D E D 80% relative humidity 1 8 6 8 6 16 hours, 120 c. 86.0 Pu p consistency, percent 3. 5 5 dry atmosphere Time, hours 1 2 2. 5 3 2 l5 Temperature, C 20 50 75 50 85 gigg y Active chlorine, percent- 7.3 1. 4 1.2 1- 20 y NaOH, ercent 2 8 0. 56 0.8

Residua chlorine, percent. Final pH The following analysis data were obtained for the pulps Y bleached by the methods referred to above:

25 pulp, although it is less than when pure oxygen was used. How- Propertles oi Treated Pulp Treatment Conditions Pulp a Pulp b Pulp c Bleaching schedule OrC/D-E-D 0'rC/DEDED CEHDED Initial brightness (SCAN), percent 9. 25 95.0 93.3 Brightness (SCAN) after aging, percen 16 hours, 80 0., 80% relative humidity 85. 0 87.8 35. 4 Post-color value 1. 01 0. 72 1. 01 16 hours, 120 0., dry atmosphere" 85. 6 89. 5 86. 5 "Post-color value 0.90 0. 4B 0. 81 Viscosity (SCAN), cmfl/g 922 950 980 Viscosity (TAPPI), cP 61 69 75 These results show that the process of this invention achieves good brightness values with only three bleaching stages after the oxygen-alkali stage. The bleaching carried out after the oxygen-alkali treatment in (a) and (b) included the C/D stage of the invention, the good delignifying and bleaching action of which enables a high brightness level to be reached by the use of a single chlorine dioxide stage after the alkaline extraction. In conventional bleaching of sulfate pulp, as shown by (c), the control, six stages are required to obtain an equivalent result. The bleaching sequence used in (b) is shown in the second column of the Table to result in a combination of brightness and viscosity, which has hitherto been impossible to achieve by conventional bleaching processes.

EXAMPLE 2 Unbleached pine sulfate pulp was used in this experiment. The analysis data of the pulp were as follows:

Kappa number 30.1 Viscosity (SCAN) 1,222 cm./g. Viscosity (TAPPl) l77 cp The following analysis data were obtained for the washed pulp:

Kappa number Viscosity (SCAN) 18.0 1,040 crn-lg.

The pulp was bleached according to the following schedule:

Treatment Conditions Stage CID Pulp consistency. 1'

ever, it is possible to compensate for this decrease in delignifi stage. An increase in temperature in the C/D stage results in the consumption of a greater amount of chemicals in a given period of time. In this Example, the reaction time has been reduced to one hour, as compared to the three hours used in Example 1, in spite of an essential increase in the amount of bleaching chemicals in Example 2. This intensified treatment in the C/D stage enables a high brightness to be obtained with only three bleaching stages, after an air-alkali treatment.

EXAMPLE 3 Unbleached spruce sulfite viscose pulp was used. The pulp had the following characteristics:

Kappa number 7.9

Extract, dichloromthane 1.41 Viscosity (SCAN) 739 cm./g. Viscosity (TAPPI) 35 op The pulp and an aqueous sodium hydroxide solution (3.1 g. NaOl-l per liter) were blended thoroughly, at 6 percent pulp consistency. The pulp was dewatered in a centrifuge to a pulp consistency of about 30 percent.

The pulp was treated in a peg shredder to obtain a fluffy pulp. This pulp was enclosed in a pressure vessel which was heated with direct steam to C. The pressure vessel was connected to an air tube and the pressure adjusted to 4 kgJcm. After 30 minutes of reaction period, the pulp was washed with water.

The washed pulp had the following analysis values:

Kappa number 3.08

Extract, dichloromethane 0.267: Viscosity. (SCAN) 675 cmllg. Viscosity (TAPPI) 28 cp The pulp was bleached according to the following bleaching schedule:

Treatment Conditions Pulp which had not been treated with oxygen was also bleached according to schedule C-E-D-l-l under the following conditions, as the control:

Stage C E D H Pulp consistency. it 3 6 5 Time. hours 1 2 Z 4 Temperature. C. 85 70 55 Active chlorine. 2.6 0.8 0.4 NaOH 7: 3.5 Residual chlorine. 2 0.2 0.2

The following results were obtained:

Example 3 Control Bleaching schedule 0,-C/D-E-H C-E-D-H Initial brightness (SCAN). i 95.0 94.7 Brightness (SCAN) after aging, 7:

16 hours, l20 C. 88.9 88.3

dry atmosphere Alkali resistance R 18. i 92.5 92.5 Extract.

dichloromethane. ii 0.17 0.31' Viscosity (SCAN).

cm./g. 610 622 Viscosity (TAPPI).

These results show that when bleaching viscose pulp according to this invention it is possible to produce a pulp having low extract content and also having analysis data which are comparable to those of conventionally bleached pulp. The bleaching cost may be reduced by about $1.00 per tone of pulp, when bleaching according to this invention.

Having regard to the foregoing disclosure, the following is claimed as the inventive and patentable embodiments thereof.

1. 1n the process for bleaching and delignifying cellulose pulps that have previously been subjected to a partial delignification treatment with oxygen under alkaline conditions, the improvement which comprises subjecting the cellulose pulp to bleaching and deligniflcation with a mixture of chlorine and chlorine dioxide at a pulp consistency within the range from about 2 to about 15 percent, and a temperature within the range from 8bOUElQimiE)l1LZQ?g. Eflime within the range from about 30 to about minutes, and then extracting the chlorinated lignin with alkali.

2. A multistage pulp bleaching and delignification process comprising a first oxygen-alkali partial delignification stage carried out with oxygen in the presence of alkali under conditions such that delignification is incomplete, and then continuing delignification in a second or subsequent stage, with a mixture of chlorine and chlorine dioxide in accordance with claim 1.

3. A process in accordance with claim 2, in which delignification is followed directly by an alkali treatment stage to extract the chlorinated lignin.

4. A process in accordance with claim 2, in which the chlorine-chlorine dioxide delignification is followed by an alkali treatment stage, and at least one treatment stage selected from the group consisting of hypochlorite treatment and chlorine dioxide treatment stages.

5. A process in accordance with claim 4, in which several treatment stages selected from the group consisting of alkali treatment, hypochlorite treatment, and chlorine dioxide treatment stages follow the delignification.

6. A process in accordance with claim 2, in which the oxygen-alkali treatment is effected at a pulp consistency within the range from about 3 to about 45 percent, a temperature from about 25 to about 150 C. for from about 10 to about minutes, with sufficient alkali to provide from about 0.5 to about 12 percent alkali by weight of the cellulosic material.

7. A process in accordance with claim 2 in which the alkali treatment is effected at a pulp consistency within the range from about 4 to about 18 percent at a temperature from about 25 to about C. for from several minutes up to about 5 hours, and with sufficient alkali to afford a pH from about 8 to about 12.5 at the end of the alkali treatment.

8. A process in accordance with claim 2, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture is within the range from about 2 to about 50 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide.

9. A process in accordance with claim 2, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture is within the range from about 10 to about 30 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide.

10. A process in accordance with claim 2, in which in the oxygen-alkali treatment, the initial oxygen partial pressure is at least about 1 atmosphere.

11. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out at a temperature of about 80 C. to about 130 C.

12. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out at a pulp consistency of from about 15 to about 35 percent.

13. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out using sodium hydroxide in an amount within the range from about 0.5 to 10 percent by weight of the cellulosic material.

14. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out in the presence of a substance inhibiting degradation of cellulose.

15. A process in accordance with claim 2, wherein the oxygen-alkali treatment is continued until the permanganate consumption of the pulp, as determined by the Kappa number according to SCAN, is reduced to between 80 and 10 percent of its initial value.

16. A process in accordance with claim 2, wherein hypochlorite is used in the alkaline treatment stage after the chlorine/chlorine dioxide stage.

zgz g UNITED STATES PATENT OFFICE I CERTIFICATE OF CQRRECTION Patent No. 3,652,388 Dated March 28, 1972 Inventor(s) CIOOD Gt 8.1.

It is certified that error appears in the above-identified patent: and that said Letters Patent are hereby corrected as shown below:

F Column 3, line 2 "'vary" should be very Column 3, line 3 "C1 both occurrences should be c1 Column 3, line 31 2 1 1 "C1 should be c1 Column 3, line 34 '2 "C1 should be Cl Column 4, line 46 E "(calculated as NaOH" should be (calculated as NaOH) Column 7, line 43 m: "Ypreferably" should be preferable I Column 9, line 30' "9.25" should be- 92.5

1st Column Column 10, line 57 "dichloromthane" should be dichloromethane Column 11, line 57 "tone" should be ton Signed and sealed this 13th dayof March 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT 'GOTTSCHALK Attesting Officer Commissioner of Patents 

2. A multistage pulp bleaching and delignification process comprising a first oxygen-alkali partial delignification stage carried out with oxygen in the presence of alkali under conditions such that delignification is incomplete, and then continuing delignification in a second or subsequent stage, with a mixture of chlorine and chlorine dioxide in accordance with claim
 1. 3. A process in accordance with claim 2, in which delignification is followed directly by an alkali treatment stage to extract the chlorinated lignin.
 4. A process in accordance with claim 2, in which the chlorine-chlorine dioxide delignification is followed by an alkali treatment stage, and at least one treatment stage selected from the group consisting of hypochlorite treatment and chlorine dioxide treatment stages.
 5. A process in accordance with claim 4, in which several treatment stages selected from the group consisting of alkali treatment, hypochlorite treatment, and chlorine dioxide treatment stages follow the delignification.
 6. A process in accordance with claim 2, in which the oxygen-alkali treatment is effected at a pulp consistency within the range from about 3 to about 45 percent, a temperature from about 25* to about 150* C. for from about 10 to about 120 minutes, with sufficient alkali to provide from about 0.5 to about 12 percent alkali by weight of the cellulosic material.
 7. A process in accordance with claim 2 in which the alkali treatment is effected at a pulp consistency within the range from about 4 to about 18 percent at a temperature from about 25* to about 130*C. for from several minutes up to about 5 hours, and with sufficient alkali to afford a pH from about 8 to about 12.5 at the end of the alkali treatment.
 8. A process in accordance with claim 2, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture is within the range from about 2 to about 50 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide.
 9. A process in accordance with claim 2, in which the proportion of chlorine dioxide in the chlorine-chlorine dioxide mixture is within the range from about 10 to about 30 percent chlorine dioxide, calculated as the amount of chlorine equivalent to the chlorine dioxide.
 10. A process in accordance with claim 2, in which in the oxygen-alkali treatment, the initial oxygen partial pressure is at least about 1 atmosphere.
 11. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out at a temperature of about 80* C. to about 130* C.
 12. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out at a pulp consistency of from about 15 to about 35 percent.
 13. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out using sodium hydroxide in an amount within the range from about 0.5 to 10 percent by weight of the cellulosic material.
 14. A process in accordance with claim 2, in which the oxygen-alkali treatment is carried out in the presence of a substance inhibiting degradation of cellulose.
 15. A process in accordance with clAim 2, wherein the oxygen-alkali treatment is continued until the permanganate consumption of the pulp, as determined by the Kappa number according to SCAN, is reduced to between 80 and 10 percent of its initial value.
 16. A process in accordance with claim 2, wherein hypochlorite is used in the alkaline treatment stage after the chlorine/chlorine dioxide stage. 